ECHINODERMATA—WATER VASCULAR SYSTEM 



421 



some of them open direct into the axial sinus. No direct communica- 

 tion between the stone canal and the axial sinus is found in adult 

 animals. 



The madrepoi'ite appears externally marked by furrows radiating from the centre 

 to the periphery (Fig. 359). In the bases of these furrows lie the apertures of the 

 pores. The pore canals, which run through the substance of the 

 madreporite to the stone canal, anastomose in definite ways, which 

 cannot here be described. 



The increase of surface of the inner wall of the stone canal 

 (Fig. 360) is of some interest. As the middle layer of connective 

 tissue takes part in the formation of the folds projecting into the 

 lumen, these folds may calcify. The simplest condition is found in 

 the Echinasteridce and Asterias fenuispina, where a projecting longi- p,,-.^ 359.— a 

 tudinal ridge is formed on the inner wall of the stone canal (Fig. quarter of the 

 360, A). In Asterina the free edge of this fold splits up into two madreporic 

 diverging lamelhu, in such a way that the transverse section is Y- or P ® °tTi' ^ 

 anchor- shaped (B). The lamellae may become coiled (species of mijens (after 

 Asterias, Pentaceros, Gymiiasteria, C). Occasionally the ridge traverses Ludwig). 

 the whole lumen of the canal as a septum (D), and then carries on 

 each surface a coiled lamella (species of Astropeden) . The whole lumen, further, 

 may be traversed by septa which, in transverse section, form a network {Luidia, 

 Culeita, species of Astropeden and Ophidiaster, F). 



Number of the stone canals and madreporitic plates. — Several madreporites 

 and stone canals (two to five and more) are not infrei^uently found in individuals 



Fig. 360.— A-F, Transverse sections through the stone canal of various Asteroids. 1, Sus- 

 pensor of tlie stone canal to the wall of the axial sinus ; 2, endothelium of the axial sinus ; 3, inner 

 epithelium of the stone canal ; 4, connective tissue portion of the wall. 



with six, seven, or more arms, belonging to species which normally have five arms. 

 There are, however, some species (having normally five or more arms) which habitu- 

 ally possess more than one madrejjorite {Asterias tapensis, A. polyplax, Ophidiaster 

 Germani, Acanthaster echinites and ^. Ellisii). On the other hand, the species of the 

 genera Solaster, Heliaster, and Luidia, w-hich normally have numerous arms, jiossess 

 only one madreporite. When more than one madreporite is present they lie, as a 

 rule, in ditferent interradii. Cases have, however, been observed in which two stone 

 canals occurred in one and the same interradius, and even in one and the same axial 



